THE HIGHER URYPTOGAMIA. 135 



tissue of the thin, lateral shoots. No trace of it is to be 

 found in the innovations* which are developed from the 

 ends of older, thinner, lateral shoots, and which, growing 

 rapidly in length and thickness, ultimately exactly resemble 

 the principal shoots in their mode of vegetation. On the 

 other hand, after the completion of the final longitudinal 

 expansion, a different mode of thickening occurs regularly 

 in the elongated cells of the axile tissue of these thin- 

 ner shoots, and also in that of the thick, principal shoots 

 and of the lateral shoots. After the completion of this 

 thickening the cell- membranes appear thick and indis- 

 tinctly stratified, their colour being yellowish-brown or 

 greenish brown, and sometimes very intense. This thick- 

 ening is most highly developed in the narrowest peri- 

 pheral cells of the axile cylinder ; it diminishes rapidly 

 in the wider, median cells. 



When, from the arching outwards of its free surface, the 

 rudimentary leaf-cell is recognisable as the mother-cell of 

 the leaf, it embraces rather more than a third part of the 

 circumference of the stem (PL XVII, figs. 3, 4, 5). At 

 this period it still lies on the immediate boundary of the 

 apical cell of the tip of the stem (PI. XVII, fig. 2). 

 When viewed from above it is clearly seen that the tan- 

 gent of its free outer margin is parallel to the tangent of 

 the arc which is represented by that one of the lateral edges 

 of the apical surface of the terminal cell of the tip of 

 the stem which is turned towards the rudimentary leaf- 

 cell (PI. XVII, figs. 4, 5). If, now, the successive divisions 

 of the terminal cell were such that each third wall were 

 parallel to the third last one (as in the diagram, PI. XVI, fig. 

 b h ), it would follow that, inasmuch as each cell of the second 

 degree produces a leaf, the leaves must be arranged under 

 one another on the stem, in three exactly parallel, longi- 

 tudinal rows. Accurate examination, however, of a termi- 

 nal bud shows that even in the youngest portions of the bud 

 this is not so. Even here also the youngest leaf- rudiments 

 have the arrangement which is characteristic of a later 



* Scbimper, 1. c, pi. xvi, f. 1. 



